Breathing apparatus for use in an underwater diving situation in atmospheres unsuitable for sustaining life have been utilized in numerous applications. These systems provide the gas supply system for a user when operating in an unsuitable environment. They are typically portable and accommodate for various adverse conditions in the unsuitable atmosphere.
In underwater diving environments, it is necessary to not only provide a source of breathing gas, but also account for the high pressures in the environment, as these breathing apparatus are utilized at various depths. As the depth increases, the environment becomes more hazardous and more care should be taken.
In conventional diving apparatus, the diver determines his "dive profile" and will mix the gases in his breathing apparatus to provide the proper gas mixtures at the desired depth, this being a well-documented procedure. Additionally, dive computers are provided for allowing the diver to monitor his dive profile, such that ascent and descent times can be monitored to insure that the diver does not develop a case of the "bends" or other well-known side effects of deep dives. However, conventional apparatus with gas mixtures provide only a minimal amount of time on the bottom, due to the fact that only a finite amount of gas can be carried with the diver. These types of systems vent the gas that is breathed into the lungs and then exhaled. Unfortunately, these types of systems allow a large amount of unmetabolized oxygen to be vented into the water.
Another type of apparatus, a re-breathing apparatus, has been developed to recycle the exhaled gas to remove carbon dioxide therefrom with a "scrubber" and then recycle the unmetabolized oxygen. Oxygen or Oxygen-enriched gas is then injected into the "scrubbed" gas to maintain the partial pressure of oxygen in the gas at a desired level, and then the mixture is passed back to the user for re-breathing. One such system is described in Great Britain Patent Application Ser. No. 9409683.1, filed May 24, 1984, upon which PCT Application No. IB 95/00396, filed May 15, 1995, was based. This is entitled "Breathing Apparatus" and was published on Nov. 23, 1995, as International Publication No. W095/31367, which application is incorporated herein by reference.
Developers of re-breather systems design the systems to maintain high efficiency, and minimize weight with concurrent minimum effort expended by the user in breathing during use of the system. Early re-breather systems were relegated to use by professionals in unsafe environmental conditions, such as diving or firefighting due to the complexity and costs of the systems, in addition to the extensive training required for the use of these systems. Although the systems are relatively simple in construction since pure oxygen is utilized, the early systems were undesirable due to the problem of oxygen toxicity, i.e., if the partial pressure of oxygen (PPO.sub.2) rises, or falls, this can be detrimental to the diver.
There are two types of re-breather systems, a closed circuit system and a semi-closed circuit system. The semi-closed circuit system is one wherein the diver is allowed to adjust the flow of oxygen-enriched gas into the breathing loop with the aid of either a calculated PPO.sub.2 or measured liters per minute. This is effected through some type of manual valve. Alternatively, the diver can have this preset at the surface, this typically being the case, wherein the diver will know the depth that the system is to be operated and it is permanently set at the surface. This, in the past, has been viewed as an advisable way to operate a re-breather, wherein the user cannot inadvertently manipulate the controls to increase or decrease the PPO.sub.2. This is quite acceptable when working at constant depths; however, when depths are varying, the oxygen must be varied and the user in a semi-closed circuit system must have some ability to change the amount of oxygen that flows into the breathing side of the system.
In closed circuit systems, a feedback mechanism is provided wherein the PPO.sub.2 is monitored and a valve is opened and closed to adjust the amount of oxygen that is introduced to the breathing side of the apparatus. These systems have typically utilized pulse-type valves, which are fully open or fully closed. These systems have a disadvantage in that, when the valve is open, the full pressure of oxygen is introduced into the breathing side of the apparatus, resulting in an uneven regulation of oxygen. Alternatively, if the system fails, it either fails open or it fails closed. If it fails open, this can be disastrous whereas, if it fails closed, this merely requires some type of backup.